The present invention relates to a biaxially-oriented opaque polypropylene film.
Biaxially-oriented, transparent or opaque polypropylene films are widely used in the packaging industry. Goods wrapped in opaque films, in particular chocolate bars or cookies, are protected against the influence of light to a far higher degree than goods wrapped in transparent films, which is of advantage in view of the desired long-term storability of the respective goods. Opaque films are films possessing vacuoles and are well-known in the art.
Disadvantages arise when opaque films are to be processed into bags or similar containers on high-speed automatic packaging machines. Usually, machines of this type are designed so that the film is guided, by exerting a tensile force, over an appropriately shaped forming tool to produce a tubing with overlapping edges from the flat film. Further process steps including sealing, filling of the tubing with the goods to be packaged and severing of the filled tubing sections, result in the individual bags.
When opaque, vacuole-containing films are guided over a forming tool in order to shape them into tubings, it is frequently observed that stripe-like markings of varying intensity appear in the longitudinal direction of the film or tubing. This adversely affects the optical appearance of the package. Text printed in the area of these undesired markings can only be read with difficulty, and industrial users that employ such films for packaging their quality products do not accept them, because an impression of inferior quality might be given to the end consumer. The markings are obviously a result of the compressibility of vacuole-containing opaque films, which is strongly increased as compared to transparent films. In the case of transparent films markings of the described type are not visible.